Magnetostratigraphy of the Middle Jurassic Sediments from Kachchh Basin , Western India

The palaeoposition of India after its breakup from Gondwana and its subsequent northward journey during the Mesozoic poses many tectonic riddles. A magnetostratigraphy pattern has been constructed for the Ammonoid rich Middle Jurassic Bathonian succession of Jumara Dome, Kachchh, western India. We present the first magnetostratigraphic results from the Ammonoid bearing exposed Bathonian sediments in Kachchh. The Jumara Dome comprised of mixed carbonatesiliciclastic succession is represented by the Jhurio and Patcham formations and siliciclasticdominating Chari Formation in the Kachchh Mainland. The Jumara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. This study is based on collection of 60 oriented samples from 13 successive levels. Both thermal and AF demagnetization investigations were carried out to isolate the Characteristic Remanent Magnetization (ChRM) directions. Palaeolatitudes were computed using the ChRM directions. Magnetostratigraphic column is prepared and correlated with Geological Time Scale (GTS) of [1] [2], and this column readily matched with GTS at M41 to below and exhibits the dominance of normal polarity in Jumara.


Introduction
Several models based on geological and geophysical evidences for the tectonic evolution of the Indian plate from its Pangean origin to the present day was proposed by [3].The evolution of the Indian plate is least understood among all the Gondwana landmasses.The first stage of rifting of Pangea took place in middle Jurassic during this time India was still a part of Gondwana.During the first phase of continental rifting Gondwana broke apart in to two equal parts, western Gondwana composed of Africa and south America and eastern Gondwana composed of India, East Antarctica, Madagascar, Australia, the Seychelles, and other microcontinental blocks, separated from Africa in the Mid-Jurassic [4] [5].
The Kachchh basin formed due to rifting and counter-clockwise rotation of the Indian plate in the Late Triassic/Early Jurassic [6].The basin is bordered by the subsurface Nagarparkar Massif in the north, Radhanpur-Barmer Arch in the east and Kathiawar uplift to the south [7].In this pericratonic rift basin [8]- [10], Mesozoic sediments ranging in age from Bajocian to Albian lay unconformably on the Precambrian basement [11].The Mesozoic sediments are exposed in the Kachchh Mainland, Patcham, Khadir, Bela, Wagad and Chorad Islands in the Great Runn of Kachchh.These sediments are dominantly represented by Siliciclastics.Magnetostratigraphic studies represent an important tool for correlation purposes since they deal with globally synchronous events such as inversions of the earth magnetic field.They set up important time constraints for biostratigraphic studies.As marine magnetic anomalies are recorded in the present-day oceanic floors, the magnetic polarity time scale is well defined from Jurassic to Recent.
The Middle Jurassic Ammonoid bearing Bathonian sedimentary successions are beautifully exposed in Kachchh Mainland and Island Belts of the Kachchh basin (Figure 1).The Bathonian sediments belong to the Patcham Formation, the oldest lithostratigraphic unit of lithostratigraphic framework of [12] [13] (Table 1).The fossiliferous Bathonian section is found the best developed at Jumara in the Kachchh Mainland (Figure 1).However, these sediments present their beautiful outcrops also in Patcham Island (Sadhara Dome, Kala Dongar and Gora Dongar range) and other localities of the Kachchh Mainland (Jhura, Nara and Habo Domes).Recent development towards the investigation of exposed Bathonian sediments at Jumara, Kachchh Mainland envisages the formulation of high resolution ammonoid biochronology [14].The sedimentary succession at Jumara has been litho-and bio-stratigraphically resolved and is expected to be potential exposure for integrated litho-biomagneto-stratigraphic studies during Bathonian interval (167.7 Ma to 164.4 Ma, GTS 2008) in India.The aim of this work is to correlate different biozonal scales in different regional settings and to contribute to the development of a standard Jurassic magnetic polarity scale in the Tethyan realm.The present study in the Jumara section is the first ever attempt to develop a magnetostratigraphic framework in the Bathonian of Kachchh, its correlation with standard magnetostratigraphic scale of [2] and also its integration with ammonoid zones of Kachchh [14] and standard Tethyan scheme [16].The Bathonian magnetic polarity stratigraphy is very ambiguous in other parts of the globe.

Lithostratigraphic Remarks
The exposed Bathonian sediments at Jumara is ~51 m thick and correspond to the part of the Patcham Formation.The base is not exposed while overlying Chari Formation bears conformable contact with these sediments (Table 1).The Bathonian outcrop in the study area is predominantly composed of relatively fine textured limestone and shale/silt/marl among which most of the limestone beds yielding ammonoids and other invertebrate fauna.The lower part of the succession (~18 m thick) is mainly consisted of alternating beds/bands of yellowish white shale/marl and yellowish gray to earthy limestone.This part of the succession is highly rich in corals besides subordinate presence of other invertebrates.The overlying fossiliferous unit (~33 m thick) is characterized by having alternating succession of grayish white/whitish marly limestone and earthy shale/silt.
The Paleomagnetic sampling was done by collecting 60 oriented block samples from 13 sites.The samples were prepared in to standard cylindrical specimens of size 2.5 cm diameter and 2.2 cm height.About 450 specimens have been obtained from 60 samples.

Methodology
The natural remanent magnetization (NRM) of all the specimens was measured on a JR-6 Spinner magnetometer (Brno, Czechoslovakia) in the paleomagnetic laboratory of the CSIR-National Geophysical Research Institute, India.Alternating field (AF) cleaning was systematically adopted using Molspin AF demagnetizer (Magnetic Measurements, UK).51 specimens were demagnetized using 17 demagnetization steps per sample, from 5 mT up to 150 mT maximum AF field.AF treatment could remove up to 20% -80% of the natural remanence.Therefore, a batch of 59 specimens was also thermally demagnetized in 19 steps from 70˚C up to 680˚C using a thermal demagnetizer (Model MMTD-80, Magnetic Measurements, UK).Demagnetization data were plotted on both orthogonal demagnetization diagrams [17] and on equal-area projections and the magnetization components were isolated using principal component analysis [18].The statistics of the paleomagnetic data obtained was conducted using [19] statistics analysis.The virtual geomagnetic pole (VGP) latitudes of the characteristic remanent magnetization components (ChRMs) from each sample with respect to the Earth's rotation axis were evaluated.The isothermal remanent magnetization (IRM) studies on 5 representative samples were analyzed using Pulse Magnetizer (Model MMPM 10, Magnetic Measurements, UK).

Paleomagnetic Results
The NRM has been measured using JR-6 spinner Magnetometer and the intensities rage between 3.88 × 10 −4 and 1.05 × 10 −5 A/M, displaying lower values in the samples collected from Jumara Section.In order to separate the secondary NRM components 51 specimens were subjected to AF demagnetization.Figure 2 shows the orthogonal projections where the vector behavior during AF demagnetization is deciphered.In most of the samples, a ChRM could be isolated in 25 -70 mT were field intervals, after the elimination of a viscous component (Figures 2(a)-2(c)).Another set of 59 specimens subjected to Thermal Demagnetization.Figure 3 shows the orthogonal projections where the vector behavior during Thermal demagnetization is deciphered.In most of the studied samples, a low temperature magnetization is unblocked below 300˚C -400˚C after removal of a viscous component at 150˚C (Figures 3(a)-3(c)).This low temperature component is parallel to the present-day geomagnetic field in in-situ coordinates and will not be considered further.A stable component with high unblocking temperature component was isolated above 300˚C and 580˚C, a ChRM can be identified in about 90% of the samples.Significant decay in the NRM intensity below 580˚C indicates the predominant contribution of magnetite, in addition to hematite, in some of the samples.It is clear that thermal demagnetization is much more efficient than AF for most of the specimens.This suggests that the magnetic mineralogy of the studied Jumara limestones is dominated by magnetite but a small amount of hematite is also present.Such magnetic mineralogy is rather constant, and similar for samples displaying a normal polarity.The characteristic magnetization vector has been calculated by means of linear regression analysis [18].The majority of the samples were clearly characterized by a two-component magnetization, examples of which are given in Figure 2 and Figure 3.The stable component, referred to as the characteristic remanent magnetization component (ChRM), shows single polarity in northerly declination with positive/negative inclination (Figure 4). Figure 5 show the isothermal remanent magnetization curves indicating that the shale samples having hematite as the remanent carrier.Table 2 gives the Paleomagnetic results from   the Jumara section of Kachchh basin.Petrography and geochemistry studies are done on the sediments of Jumara dome [20] emphasized that the sandstones/shales were deposited on a passive margin of the stable intracratonic basin.The results of paleocurrent direction on the Jurassic sandstones/shales of the Kachchh Basin suggests that sand detritus was mostly derived from the Aravalli range situated to the northeast, east and southeast of the Kachchh basin and from the Nagar parker massif situated to north and northwest of the Kachchh basin.

Magnetostratigraphy
A magnetostratigraphic sequence has been established by plotting the Virtual Geomagnetic Pole (VGP) of the ChRM component of each sample as a function of its stratigraphic position.Figure 6 is the plot of the VGP Latitudes against the Lithocolumn.A clear magnetic zonation of the sedimentary sequence at Jumara is defined.Interestingly the Jumara section has not recorded any reverse polarity.Entire section has recorded the normal polarity, there are some indications at lower part of the section where hard shales were sampled have shown southern hemisphere declinations with positive inclination (2 -3 specimens), however we have not considered these results in the statistics.The Magnetostratigraphy column is correlated with the standard Geological Time Scale (GTS) of Ogg et. al., 2008.The Bathonian period in GTS range from 167.7 -164.7 which is having 2 -3 major reversals where the Jumara section has not recorded this polarity.Moreover there is no certain major zones exist in GTS below M41 and it is difficult to correlate this section with standard GTS.However, with the available literature on Paleontology and fossil content of this section the MPS can be placed at middle Bathonian in GTS at M41 or above/below.

Conclusion
This paper presents the first magnetostratigraphic dating of the fossil-bearing continental sediments of the Jumara section, Kachchh basin, western India.From the magnetic mineralogy study it is inferred that the primary remanent magnetization resides in magnetite as well as hematite.The erected magnetic polarity stratigraphy column indicates that the Jumara section correlates well with standard GTS of [1] [2].Based on this correlation the MPS can be placed at middle Bathonian to lower Callovian in GTS at M41 or below/above.

Figure 1 .
Figure 1.Geological map of the Kachchh, western Gujarat, India showing the study area in red rectangular shape [15].

Figure 2 .
Figure 2. Examples of orthogonal vector diagrams during AF demagnetization.(a) Vector migration diagram; (b) orthogonal projection.Closed (open) symbols are projection onto the horizontal (vertical) plane; (c) intensity decay curves.

Figure 3 .
Figure 3. Examples of orthogonal vector diagrams during thermal demagnetization.(a) Vector migration diagram; (b) orthogonal projection.Closed (open) symbols are projection onto the horizontal (vertical) plane; (c) intensity decay curves.

Figure 4 .
Figure 4. Mean ChRM directions along with grand mean for Jumara section, Kachchh basin.

Figure 6 .
Figure 6.Plot of Lithostratigraphy against the sampling sites.Magnetostratigraphy of the Jumara Section and its correlation with standard GTS of [1] [2].

Table 1 .
Lithostratigraphic framework of Mesozoic sedimentary succession of Kachchh, western India and their ages (after Pandey et al. 2012).